1. Field of the Invention
The present invention relates to a method of driving a liquid crystal display, and particularly to a method of driving, in a flickerless manner, a liquid crystal display employing liquid crystal dots arranged in a matrix.
2. Description of the Prior Art
As is known, a liquid crystal display (LCD) has advantages such as low power consumption and portability. The LCDs are widely used, therefore, for portable calculators and watches to display characters. With development of office automation, i.e., automation of business machines, high performance LCDs are required to realize highly integrated business machines. To meet the requirement, a thin film transistor liquid crystal display (TFTLCD) employing thin film transistors (TFTs) as switching elements of pixels has been developed and produced.
FIG. 1 shows a conventional TFTLCD. The TFTLCD comprises pixels Pll to Pnm arranged in a matrix. The pixels are connected to signal lines Xl to Xm and scan lines Yl to Yn. A signal electrode driving circuit 1 and a scan electrode driving circuit 2 turn on the pixel Pnm and provide a display signal to the pixel.
FIG. 2 is an equivalent circuit of one of the pixels of the TFTLCD. The circuit comprises a liquid crystal dot 3nm and a switching element 4nm, i.e., the TFT. This TFT is usually made of amorphous silicon, polysilicon, silicon surfer, etc.
To drive the TFTLCD of FIGS. 1 and 2, the scan electrode driving circuit 2 provides a scan pulse through the scan line Yn to the liquid crystal dot 3nm. According to a display pattern, the signal electrode driving circuit 1 provides a signal voltage through the signal line Xm. The pulse through the scan line Yn turns on the TFT 4nm, and the signal voltage charges a capacitor 5nm. After the TFT 4nm is turned off, the capacitor 5nm holds the charged voltage until the TFT 4nm is again turned on. The voltage held in the capacitor 5nm is applied to the liquid crystal dot 3nm to display a dot.
FIG. 3 is an equivalent circuit of the TFTLCD of FIG. 1. In FIG. 3, the TFTLCD comprises signal lines Xl to Xm; scan lines Yl to Yn; TFTs 4ll to 4nm disposed at intersections of the signal and scan lines; capacitors 5ll to 5nm connected to the TFTs, respectively; liquid crystal dots 3ll to 3nm connected to the TFTs, respectively; and a common potential 6 to which one ends of the capacitors and liquid crystal dots are connected.
An operation of the TFTLCD of FIG. 3 will be explained with reference to FIGS. 4a to 4c.
The signal electrode driving circuit 1 applies a voltage signal Vsm having time/voltage characteristics of FIG. 4a to the signal line X (Xl, . . . , Xm). The scan electrode driving circuit 2 applies a gate voltage Vgn of FIG. 4b to the scan line Y (Yl, . . . , Yn). As a result, a drain voltage VD of FIG. 4c for a selected field is applied to a liquid crystal dot disposed at an intersection of the lines X and Y. At this time, an "ON current" Io is expressed as follows: EQU Io=Cox.multidot..mu.(W/L)(VD-VsN) {Vgn-Vth-(VD+Vsm)/2} (1)
where
Cox=gate insulation film capacity PA0 .mu.=mobility PA0 Vth=threshold voltage PA0 W=TFT channel width PA0 L=channel length PA0 Cs=storage capacitance PA0 CLc=liquid crystal dot capacitance PA0 Cgd=capacitance between gate and drain PA0 Cpd=capacitance between adjacent signal line and liquid crystal dot PA0 (1) Insufficient TFT "ON current" PA0 (2) Leakage of gate voltage due to gate/drain capacitance of TFT PA0 (3) TFT "OFF" current. PA0 ly=vertical pixel pitch PA0 n=0, 1, 2, . . . PA0 Tf=field period
As is apparent from the equation (1), the "ON current" is insufficient when the voltage Vsm is positive, so that a waveform of the driving voltage VD may be asymmetrical on positive and negative sides as shown in FIG. 4c. This may cause flickers.
Each liquid crystal dot 3nm reacts to an effective value of the driving voltage, which varies for each field across a voltage level Vcom. Accordingly, the transmission, i.e., intensity of each liquid crystal dot differs for each field, thereby causing the flickers.
As is understood from FIG. 2, when the gate voltage Vgn is turned off, the voltage VD leaks to the liquid crystal dot through a parasitic capacitance Cgd between the gate and drain and decreases by .DELTA.Vp, which is expressed as follows: ##EQU1## where Cds=capacitance between signal line and drain
This voltage change .DELTA.Vp appears for every field to cause the flickers.
In addition to the above two factors, there is another factor that causes the flickers, i.e., an "OFF current" of the TFT. The "OFF current" changes in response to a gate/source voltage Vgs of the TFT to produce a difference (.DELTA.V.sup.+ off-.DELTA.V.sup.- off) between the positive and negative sides of the pixel voltage VD, thereby causing the flickers.
Consequently, there are the following three factors that cause the flickers:
As explained above, due to the insufficient characteristics of the switching element (TFT), an effective voltage applied to each pixel differs depending on the positiveness and negativeness of a driving voltage, so that, when a normal field inverting operation is carried out, plane flickers of 30 Hz may occur.
To reduce the plane flickers, a method of driving a liquid crystal display by inverting the polarity of a driving voltage within a frame has been proposed. This method converts the plane flickers into line flickers or into very small plane flickers such as pixel flickers, thereby reducing visible flickers.
FIGS. 5a to 5c show conventional flickerless driving techniques disclosed in Japanese Laid-Open Patent No. 60-156095 which inverts the polarity of a signal line, Japanese Laid-Open Patent No. 60-3698 which inverts the polarities of signal and scan lines, and Japanese Laid-Open Patent No. 60-151615 which inverts polarities for each scan.
FIG. 5a shows the field inverting technique in which polarities are inverted for each field.
FIG. 5b shows the scan inverting technique in which polarities are inverted for each scan. The inversion is carried out not only for every frame but also within a frame, thereby alternately driving each pixel.
FIG. 5c shows the column inverting technique in which the polarities of signal lines (FIG. 3) are alternately inverted. Similar to the line inverting technique, the polarities are inverted between frames to convert the plane flickers into column flickers.
It has been confirmed experimentally that the inframe inverting technique such as those of FIGS. 5b and 5c can theoretically and practically reduce the plane flickers of each frame less than a visible level by balancing intensity of each frame.
The conventional techniques of FIGS. 5a to 5c produce, however, visible horizontal and vertical stripes. This will be explained.
The driving technique of FIG. 5a inverts polarities field by field, so that the technique is not effective in reducing the plane flickers.
The driving method of FIG. 5b inverts polarities for every scan, so that the technique is effective in reducing the plane flickers but produces visible horizontal stripes corresponding to scan lines. Particularly when a motion shot by moving a camera, i.e., a so-called pan is displayed on a screen and when the eyes of an observer follow the motion on the screen, the horizontal stripes are especially visible. A speed of the eyes in a vertical direction on the screen is expressed as follows: EQU Ve=(2n-1)ly/Tf
where
If the speed of the eyes coincides with a movement of a horizontal stripe caused by the inverting operation in a frame, the horizontal stripe is seen as if it is stopped. Consequently, the horizontal stripe is clearly seen on the screen. This is not preferable.
The driving method of FIG. 5c inverts the polarity of each signal line, so that the technique is effective in reducing the plane flickers but produces visible vertical stripes. This is because a color signal G among color signals R, G and B is most perceivable. As shown in FIG. 5c, therefore, a vertical stripe of color G is formed. Similar to the case of FIG. 5b, when the eyes of an observer move horizontally to follow a motion on a screen, the vertical stripe may particularly be visible.
Conditions that make the vertical and horizontal stripes more visible will be considered.
FIGS. 6a and 6b show experimental results of visibility/discrimination threshold characteristics with respect to a moving line. As is apparent in the figures, a high-speed motion provides low band-pass spatial frequency characteristics, and a low-speed motion provides band-pass characteristics having maximum sensitivity at 3 cycle/deg. The maximum sensitivity of a slightly moving motion is higher than that of a stopped motion. In any case, a contrast and spatial frequency determine a visible range, and the conventional flickerless driving techniques operating on the present TFT characteristics produce visible vertical and horizontal stripes.